Regardless of its place of origin, crude oil recovered from the earth is a broad mixture of different sizes and types of hydrocarbons, including for example C2-C50, or even higher, hydrocarbons that may be straight chain, branched or aromatic molecules. Different sizes and types of hydrocarbons have different properties, including boiling points, ignition temperatures, viscosities, and densities, among others, which makes them suitable for different uses. Accordingly, crude oil is typically first refined by atmospheric distillation to separate the crude oil into portions or “fractions,” including light distillates, middle distillates, and heavy distillates and residuals. Each fraction is still a mixture of hydrocarbons of various types and sizes but, after separation by atmospheric distillation, each fraction contains a mixture of hydrocarbons having a narrower range of properties than the original crude oil mixture.
For example, the light distillate fraction obtained from atmospheric distillation of crude oil may include primarily C2-C10 hydrocarbons, which are the smaller, lighter hydrocarbon molecules from the original crude oil. Light distillates typically have a boiling point of up to about 177° C. (about 350° F.) and are useful, for example, as liquid petroleum gas, gasoline, and naphtha. Middle distillates may include primarily C8-C23 hydrocarbons, which are the mid-sized hydrocarbon molecules from the original crude oil. Middle distillates generally have a boiling point range of from about 149 to about 371° C. (about 300 to about 700° F.) and are useful, for example, as kerosene, diesel fuel, jet fuel, and light fuel oil. The heavy distillate fraction and residual products derived from atmospheric distillation of crude oil include primarily C18 and higher hydrocarbons, which include the largest, heaviest hydrocarbon molecules from the original crude oil. Heavy distillates generally have a boiling point of at least about 260° C. (about 500° F.) and are useful, for example, as heavy fuel oil (HFO), lubricating oils, wax and asphalt.
Each of the distillate fractions described above may, in fact, contain very small amounts of hydrocarbons outside the aforesaid ranges, without altering the general characteristics of the distillate fractions. Furthermore, the aforesaid boiling point range for each distillate fraction is approximate and may vary slightly depending on the original composition of the crude oil and the desired apportionment of its component hydrocarbons among the fractions, which may in turn be influenced by local industry standards, market demand, and government regulation. Also, the operating conditions under which atmospheric distillation is performed can shift the quantity and particular property ranges of each fraction that is produced. Due to this variability, there is some overlap of the property ranges among the various fractions.
Any of the foregoing distillate fractions may be further refined to produce additional fractions having even narrower ranges of hydrocarbon types and sizes. For instance, heavy distillate and residual products may be further distilled, under vacuum conditions, to further refine the components of this heaviest fraction of crude oil. One product of vacuum distillation is vacuum gas oil (VGO), which may include primarily C20-C50 hydrocarbons and have a boiling point of from about 343 to about 552° C. (about 650 to about 1025° F.). Another example would be further refinement of a middle distillate fraction to produce a typical home heating oil (light diesel) fraction containing primarily C10-C20 hydrocarbons and having a boiling point range of from about 249 to about 349° C. (about 480 to about 660° F.).
Currently, there is greater market demand for light and middle distillates and the products derived therefrom than for the heavier distillation products. Consequently, heavy distillates and residuals, as well as VGO, and other heavy hydrocarbon feedstock are often converted to lighter hydrocarbons, such as those contained in the light and middle distillates. This conversion may, for example, be accomplished by processes such as thermal cracking and catalytic cracking. “Cracking” refers to processes during which the larger hydrocarbon molecules are “cracked,” i.e., broken apart, into lighter, smaller hydrocarbon molecules.
In fluid catalytic cracking (FCC) processes, preheated fine particulate catalyst solids are fed to the bottom a vertical column or tube typically referred to as a “riser,” along with a preheated heavy hydrocarbon feedstock, such as VGO, which is vaporized upon contact with the catalyst in the riser. Initially, the large hydrocarbons of the heavy hydrocarbon feedstock are converted to cracked hydrocarbon products comprising primarily middle-sized hydrocarbons, with some smaller light hydrocarbons also being formed. As the fluidized catalyst, vaporized feedstock and cracked hydrocarbon products continue to ascend in the riser, the cracking reaction continues, consuming the heavy hydrocarbons and also converting some of the middle-sized hydrocarbons to smaller light hydrocarbons.
Just as with atmospheric distillation, the operating conditions under which the FCC process is performed can shift the quantity of middle and light hydrocarbons that are produced. For example, lower temperatures or shorter residence times will reduce cracking of the middle hydrocarbons to light hydrocarbons, to provide more of the middle hydrocarbons in the final product but less of the light hydrocarbons, and may also result in lower conversion of the heavy hydrocarbon feedstock in a single pass through the riser. Likewise, higher temperatures or longer residence times will provide nearly 100 percent conversion of the heavy hydrocarbons, as well as increased yield of light hydrocarbon products, but will also decrease the amount of middle hydrocarbon products. Thus, FCC process operating conditions, such as temperature and residence time, may be selected to maximize production of the preferred products, whether middle distillate products, or light distillate products. The ability to control FCC processes and the hydrocarbon profile of the products therefrom is of high importance. As there is a great demand for middle distillates, a need exists for methods and systems that maximize the amount of middle distillate products produced during FCC.
Accordingly, it is desirable to provide methods and systems for increasing the production of preferred fractions, such as middle distillates, from heavy hydrocarbon feeds in a fluid catalytic cracking process.